Internal pressures - Wind Loading of Structures - page 152

6
Internal pressures
6.1 Introduction
Internal pressures induced by wind can form a high proportion of the total design wind
load in some circumstances—e.g. for low-rise buildings when there are dominant
openings in the walls. On high-rise buildings, a critical design case for a window at a
corner may be an opening in the wall at the adjacent wall at the same corner—perhaps
caused by glass failure due to flying debris.
In this chapter, the fundamentals of the prediction of wind-induced internal pressures
within enclosed buildings are discussed. A number of cases are considered: a single
dominant opening in one wall, multiple wall openings and the effect of background wall
porosity. The possibility of Helmholtz resonance occurring is also discussed.
6.2 Single windward opening
We will first consider the case of a dominant windward wall opening—a situation which
often arises in severe wind storms—often after the failure of a window glass due to flying
debris. In a steady flow situation, the internal pressure will quickly build up to equal
external pressure on the windward wall in the vicinity of the opening—there may be
some oscillations in internal pressure (Section 6.2.4), but these will die out after a short
time. However, when a building is immersed in a turbulent boundary-layer wind, the
external pressure will be highly fluctuating and the internal pressure will respond in some
way to these fluctuations. As there is only a single opening, flow into the building
resulting from an increase in external pressure will cause an increase in the density of the
air within the internal volume; this, in turn, will produce an increase in internal pressure.
The pressure changes produced by wind are only about 1% of atmospheric pressure
(1000 Pa compared to atmospheric pressure of about 100,000 Pa) and the relative density
changes are of the same order. These small density changes can be maintained by small
mass flows in and out of the building envelope, and consequently the internal pressure
can be expected to respond quite quickly to external pressure changes, except for very
small opening areas.
6.2.1 Dimensional analysis
It is useful to first carry out a dimensional analysis for the fluctuating internal pressures,
resulting from a single windward opening to establish the non-dimensional groups
involved.